How To Conduct A Sky Quality Survey

Two hikers taking SQM measurements of the night sky on a trail in Saguaro National Park, Arizona. Photo by Jeremy White, National Park Service.

A thorough sky quality survey is essential to a successful application to the International Dark Sky Places Program.

There are a variety of ways to approach making a survey. These include:

Sky Quality Meter Survey

Bortle Scale Interpretation

Photographic Evidence

SKY QUALITY METER SURVEY

The Unihedron Sky Quality Meter (SQM) is the most widely used device for taking scientific-quality measurements of sky brightness. This small, battery-powered device is available directly from the manufacturer for US$119.99 plus shipping and handling.

The SQM is easy to use: simply hold the device above your head, pointing the photometer at zenith, and click the button. The screen will then display the sky brightness at that point in ‘astronomer’ units (magnitudes per square arcsecond). This unit is somewhat counterintuitive in that the higher the number, the darker the sky is. Take at least six measurements per location per visit, discarding the first measurement. Report all of the measurements.

Never take a measurement directly underneath a light source or anything that might block the clear sky. Always take measurements with clear open sky, cloudless, to prevent any sky glow measurements that will deter the accuracy of the device. Also, always take measurements on clear nights when the Moon is below the horizon. Measurements taken while the Moon is visible will not accurately reflect the natural darkness of your location.

Similarly, keep in mind the impact of twilight on your measurements. Only take readings under conditions of ‘astronomical darkness‘, meaning that the Sun is at least 18º below the local horizon. Good sources of information for times of the end and beginning of astronomical twilight at your location on any given night can be found here and here. (Note that for these calculators you may need to know your latitude, longitude, and local timezone offset from GMT.)

Using this device a “grid” of the locations may be made. It is essential to include areas that will be the darkest and the brightest to achieve a comprehensive survey of the location. The chart below is useful for visualizing how to plan the number and location of a set of measurements. Include measurements of the borders, active areas, and presumed darkest areas of the park. This data can then be organized into a map showing approximate location of measurements and a table. An example table is also shown below.

SQM Average

Location

Reserve Section (for Reserves only)

Notes

21.0

Border

Buffer Zone

20.5

Active Area

Critical Buffer

Brightest Area

21.5

Darkest

Core Zone

Darkest Area

The program guidelines discuss how dark the sky must be to achieve each tier designation (Gold, Silver, or Bronze):

BORTLE SCALE INTERPRETATION

The Bortle Scale works to estimate sky brightness and interpret how light pollution is affecting your view of night sky phenomenon. The easiest way to interpret the Bortle Scale is an easy flow chart. Follow the questions to reveal what Bortle Class your skies fall under. This may need to be done at different locations in the location if sky brightness varies, for instance, you may have a Bortle Class between 3 and 4. The lower the number is, the better sky quality.

This method is less quantitative than the Sky Quality Measurement. An application using only this method to document Sky Quality is not as strong as an application using the SQM method or using two or more methods.

Steve Owens, International Dark Sky Places Committee member, is credited with the flow chart below. Learn more about it on his Dark Sky Diary.

PHOTOGRAPHIC EVIDENCE

Like the Bortle Scale method this method is less quantitative than the Sky Quality Measurement. An application using only this method to document Sky Quality is not as strong as an application using the SQM method or using two or more methods.

However, it can be very useful to support the Bortle Scale method and document specific nighttime phenomenon. For example, imaging the Milky Way, the Zodiacal light, and the Gegenschein make for a much stronger use of the Bortle Scale.

When including images also include a breakdown of the camera details used to take it, i.e. exposure time, focal length, ISO, etc… See the example below.